The present invention relates to a secondary air supply system for an engine unit, particularly for a two-stroke cycle engine of a vehicle.
In a known four- or two-stroke cycle engine unit of a vehicle such as an automobile or motorcycle a piston of a piston-cylinder assembly performs a reciprocal motion in a cylinder and an exhaust gas exhausted from an engine is fed into an exhaust pipe to which a catalyst is provided for the purification of the exhaust gas through an oxidation reaction. This oxidation reaction is carried out by supplying air into the exhaust pipe called secondary air for the engine unit, in comparison with an air to be mixed with a fuel as a primary air. The secondary air is generally fed into the exhaust pipe through a one-way valve by the utilization of a pressure variation, i.e. pulsation, of the exhaust gas in the exhaust pipe to thereby suck the secondary air thereinto, or the secondary air is supplied by the actuation of a pump means attached to the engine unit.
In the former type, in which the one-way valve is incorporated, such secondary air supply system can be constructed relatively easily with no increased cost, but in the employment of such type structure, less intake air is fed at a time of an engine high-power operation period, i.e., engine high revolution speed operation, at which time voluminous exhaust gas is discharged, resulting in insufficient purification of the engine exhaust gas.
In the latter type, in which a pump means is employed, it may be possible to supply the secondary air in proportion to the engine exhaust gas amount, but this will result in an output power loss of the engine, and further, the mounting of such pump means requires modification of or design changes to an existing exhaust gas exhausting unit.